The present invention generally relates to hydrostatic sliding bearings, including hydrostatic sliding bearings suitable for use in positive displacement machines.
Positive displacement pumps and motors, such as axial and radial piston machines, generally comprise an array of pistons that reciprocate within a cylinder block. In axial piston machines, the piston-cylinder combinations are parallel and arranged in a circular array within the cylinder block. An inlet/outlet port is defined at one end the cylinder block for each individual piston-cylinder combination, such that a fluid can be drawn into and expelled from each cylinder through the port as the piston within the cylinder is reciprocated. The end of the cylinder block containing the inlet/outlet ports defines an axial sliding bearing surface that abuts a surface of a valve plate, while the opposite end of the cylinder block is connected to a drive shaft for rotation of the cylinder block. The valve plate defines an inlet opening and an outlet opening that are sequentially aligned with the inlet/outlet of each cylinder, so that fluid is drawn into each cylinder through the cylinder's inlet/outlet port when aligned with the valve plate inlet opening and expelled from each cylinder through the cylinder's inlet/outlet port when aligned with the valve plate outlet opening. The mating surfaces of the cylinder block and valve plate are axial sliding bearing surfaces separated by a film of the fluid being worked on by the machine, defining a hydrostatic axial sliding bearing that is subjected to an axial load applied by the cylinder block to the valve plate. In addition to carrying this axial load, the axial sliding bearing must also minimize fluid leakage between the block and valve plate. Consequently, the axial sliding bearing has both a bearing function and a sealing function, which differentiates the hydrostatic axial sliding bearing from typical bearing applications that have only a load-bearing function.
While the axial sliding bearing of a positive displacement machine is often referred to as a hydrostatic bearing, it is well known that there are hydrodynamic effects present that influence the load-bearing and sealing functions of the bearing as the cylinder block rotates relative to the valve plate. Nonetheless, for convenience sliding bearings of positive displacement machines and similar applications will be referred to herein simply as hydrostatic bearings.
One end of each piston protrudes from the cylinder block and is coupled with a stationary swash plate inclined to the axis of the cylinder block, causing the pistons to reciprocate within the cylinder block as the block is rotated relative to the swash plate. The stroke length of each piston, and therefore displacement of the piston-cylinder combinations, can be made variable by changing the inclination (cam angle) of the swash plate. To provide this capability, the protruding end of each piston may be configured to have a ball-and-socket arrangement. The socket portion of this arrangement, or slipper, may have a planar surface that bears against the swash plate. The spherical mating surfaces of each piston-slipper combination and the planar mating surfaces of the swash plate and each slipper define axial sliding bearing surfaces, which are separated by a fluid film formed with, for example, the fluid being worked on. The resulting hydrostatic axial sliding bearings transfer the piston force to the swash plate during relative motion between the slipper and swash plate.
Axial sliding bearing surfaces similar to those described above can also be found in other machines, including other positive displacement machines such as bent axis piston machines, radial piston machines, vane type machines, gear machines, screw-type machines, etc.
The efficiencies of machines with sliding bearing surfaces are dependent on the torque losses attributable to each sliding bearing surface. For positive displacement machines, efficiencies are also dependent on power losses attributable to fluid leakage at the axial sliding bearing surfaces defined by the cylinder block and valve plate. Designs for axial sliding bearings are widely known and described in the literature. For example, descriptions of axial sliding bearings for different positive displacement machines can be found in Ivantysyn J. and Ivantysynova, M., Hydrostatic pumps and motors, New Delhi. Akademia Books International, ISBN-81-85522-16-2 (2001). Design principles and calculation methods typically assume that the gap height between the sliding bearing surfaces is uniform. Using the axial piston machine described above as an example, the sliding bearing surfaces of the planar mating surfaces of the cylinder block and valve plate and the planar mating surfaces of the slippers and swash plate are assumed to be parallel, and the sliding bearing surfaces of the spherical ball-and-socket mating surfaces of the pistons and slipper are assumed to be perfectly spherical and concentric. For manufacturing purposes, absolute deviations of flatness are defined in ranges of micrometers. It is also common practice to assume ideally smooth surfaces within the sizing process, and to allow a minimum surface roughness for manufacturing purposes, typically less than a one micrometer Ra and more typically in a range of thousandths to tenths of a micrometer, requiring an abrasive finishing operation such as lapping. Common design principles and calculation methods further assume that the fluid film between the sliding bearing surfaces is of constant thickness.
A disadvantage of the above commonly-used design approach is that, in the event of an asymmetrical bearing load during relative motion between the sliding surfaces, the surfaces will incline with respect to each other and form a gap of variable height, leading to hydrodynamic effects. In the case of axial sliding bearings used in positive displacement machines, inclination of the surfaces can lead to conditions with very low gap heights on one side and very high gap heights on the opposite side. Such conditions increase friction in areas of relatively small gap heights and increase leakage in locations of relatively large gap heights, resulting in increased power losses of the machine and reduced machine efficiency. This problem is common for all asymmetrically-loaded axial sliding bearings that have a sealing function in addition to a sliding bearing function.
In view of the above, there is a desire to minimize power losses, resulting from friction and/or fluid leakage, in machines with hydrostatic axial sliding bearing surfaces.